Approximately one-third of the entire energy consumption of the sugar industry is spent in drying the pressed pulp from sugar processing. This amounts to about 2 kg of standard coal for each 100 kg of sugar beet. Pulp, as well as molasses, are by-products of sugar making and are used singly or in combination as feeds. The molasses may be added to the pulp before or after drying. The drying of the pulp to a water content of less than 12% is required for the storability and transportability of the pulp.
As a rule, wet pulp leaves the extraction plant with a dry-solids content of less than 10%, of which about 1% is sugar, and is pressed mechanically in pulp presses to a dry-solids content of about 20%. The pressed-out water thus obtained contains about 1% sugar and about 0.3% of other organic and inorganic compounds and is fed back into the extraction process to increase the sugar yield and to avoid detrimental wastes.
Independently of the design of the presses used, the efficiency of the mechanical dehydration depends on the following factors: conditions of vegetation of the beet, particle size and shape of the sugar beet cossettes, operating conditions during extraction (temperature, residence-time, pH value) and the operating conditions of the press.
Accordingly, in large-scale plants, as a rule, a dry-solids content of the pressed pulp of 18 to 23% has been achieved so far. While dry-solids contents up to 26% are possible for extreme operating conditions, for instance, low pH values in the extraction and highly decreased press output, the mechanical dehydration becomes uneconomical in comparison with thermal drying.
It is known to improve the dry matter of pressed pulp by adding salts of calcium or aluminum, by means of the diffusion-water, to the sugar extracting system (D. Becker et al, Zucker 14, 343-5, 1956, and D. Becker, Zucker 17 394-7, 1958). However, in this procedure, about 90% of the anions (chloride or sulfate) added to the diffuser together with the salts reach the raw sugar juice in the form of their alkali salts, pass through the juice purification, and thereby increase the sugar loss in the molasses. Therefore, this process is uneconomical in most cases.
To date, the industrial realization of a proposal to mix the carbonation sludge obtained in the sugar industry with the wet pulp and to increase in this manner the pressing properties of the pulp (F. Teschner, Die Nahrung 20, 817-821, 1976), has failed on account of the too-low solubility of calcium carbonate in this system and because of the impossibility of achieving the residence times required to achieve this end.
It is a further conventional practice to mix pressed pulp with molasses which enter the mixing procedure with a dry-solids content exceeding 80%. This pulp is directly fed to a thermal drying stage.
It is furthermore known to subject such a molasses-and-pulp mixture to another mechanical pressing operation and possibly to a subsequent drying procedure because the molasses with their high dry-solids content mix with the residual water in the pulp with very low dry-solids content and thereby a dry matter content in the surrounding liquid of about 10% is obtained. This modifies the osmotic relations between aqueous phase and hydratized plant fiber and, most of all, increases the liquid volume, which can again be pressed out. The resulting dry-solids content of the liquid phase is essentially dependent on the amount of added molasses. Energy conservation can be achieved in that the liquid mixture pressed out in the second pressing stage consisting of molasses and residual water can be evaporated in a single or multi-stage evaporation plant at little cost in energy or waste heat, whereby the amount of water to be evaporated in the directly heated pulp drying stage is decreased.